In general, according to vector control, it is possible to control a magnetic flux current component and a torque current component independently from each other by separating the current of an AC electric motor into a magnetic field (magnetic flux) direction component and a torque direction component perpendicular to the magnetic field, thereby to control the generated torque instantaneously, as in the case of a DC electric motor (see, for example, a First Non-patent Document).
In a known vector control apparatus for an induction motor, the amplitude, frequency and phase of a voltage supplied to the induction motor are calculated by using a circuit constant of the induction motor. In this case, when there exists an error between a circuit constant set in the vector control apparatus for an induction motor and an actual circuit constant of the induction motor, it becomes impossible for the vector control apparatus to maintain its vector control state, so an actual torque generated by the induction motor does not coincide with a torque command value from a system side. In addition, when the torque command value changes transitionally, the torque generated by the induction motor might cause overshoot or the like so the reaction or operation of the induction motor becomes vibratory, thus resulting in the deterioration of the torque control performance.
A secondary resistance set value among circuit constants of the induction motor can be said as follows. That is, a secondary circuit (rotor) is generally made of a copper alloy, so the actual resistance value thereof changes in accordance with the operating condition of the induction motor and a temperature change in the ambient atmosphere. Thus, there will be caused an error between the secondary resistance set value set in the vector control apparatus of the induction motor and the actual resistance value, thereby influencing the torque control performance. Accordingly, a construction or arrangement to correct such an error between the set value and the actual value is employed in many cases (see, for example, a First Patent Document).
Moreover, a mutual inductance as one of induction motor constants changes to a limited extent due to a temperature change, but an error or difference between the set value and the actual value thereof provides an influence on the steady-state torque characteristic of the induction motor.
In cases where the set value contains an error in a sense larger than the actual value, the torque generated by the induction motor becomes smaller than the torque command value, whereas in case where the set value contains an error in a sense smaller than the actual value, the torque generated by the induction motor becomes larger than the torque command value.
For example, with a vector control apparatus for an electric railway vehicle, if there is such a torque error, a problem will arise that the acceleration and deceleration of the vehicle can not be controlled in a manner as intended by an operator. Since the torque control performance is influenced in this manner, it is necessary to make the set value and the actual value coincide with each other as much as possible.
Accordingly, when the mutual inductance can be measured from a primary current of the induction motor by running the induction motor under no load, the measured value can be used as the set value.
However, actual measurements are difficult in inductance motors of built-in use in which a no-load condition can not be created, or in large-scale induction motors for installation on vehicles. Thus, in such cases, a design calculation value is adopted as the set value of the mutual inductance, so there will be an error or difference between the set value and the actual value of the mutual inductance. As a result of such a difference, the torque output of such an inductance motor does not coincide with the torque command value thereof, generating an error therebetween.
Further, much consideration is not given to the correction of the set value of the mutual inductance.
[First Non-Patent Document] “Vector Control of AC Motor” by Yoshitaka Nakano, published by Nikkan Kogyo Shinbun Co., on Mar. 29, 1996
[First Patent Document] Japanese Patent Application Raid-Open No. H06-38574